The invention relates to a method for detecting superconductivity. In particular, this invention uses resistance measurements at various temperatures and magnetic fields to determine the superconductive state.
Microwave absorption is one of several methods used to infer the presence of superconductivity in materials. This type of measurement has the advantage of being able to detect superconductivity in samples which are discontinuous. Microwave methods can also be very sensitive, permitting study of material samples which are very small. Most microwave methods involve the observation of the Q of a microwave resonator. More particularly, a waveguide cavity is used in a bridge in which reflected power from the cavity is recorded. Variations in the reflected power provide a measure of corresponding changes in the Q of the cavity. A sample located within the cavity affects the Q of the cavity by absorption of energy from the cavity fields. In applying this technique to the study of superconductors, the reflected power is measured as a function of the temperature of the sample which produces a sigmoid curve as shown in FIG. 1. This curve is similar to that produced in dc resistance measurements, indicating the transition to the superconducting state. However, many non-superconductive compositions, such as vanadium dioxide, also produce the same sigmoid curve.
Another technique useful in the determination of superconductivity uses a four-point probe to measure resistance. This technique is time consuming and requires a continuous sample, to which four leads are fixed.
U.S. Pat. No. 4,185,237 relates to a method of measuring the density of a material from a derivative absorption spectrum in a magnetic field sweep. Electron spin resonance (ESR) techniques are used wherein a sample is placed in a tube and set in an ESR cavity resonator. An ESR spectrum is measured by continuously sweeping the magnetic field while the microwave frequency to the cavity remains unchanged. Derivative peaks of a phase sensitive detector output are observed.
U.S. Pat. No. 3,805,119 discusses determining the characteristic of a superconductor by using the superconductor as the coil of a helical resonator which is mounted with its axis perpendicular to a variable d.c. magnetic field.